1. Field of the invention
This invention relates to a semiconductor laser array device in which a plurality of semiconductor lasers are disposed in a parallel manner with an optical phase coupling between the adjacent semiconductor lasers.
2. Description of the prior art
Semiconductor laser devices having a single semiconductor lasing filament structure can only produce 80 mW laser beams at their best. In order to produce high optical output power, semiconductor laser array devices, in which a plurality of semiconductor lasers are disposed in a parallel manner on a single substrate to achieve an optical phase coupling between the adjacent semiconductor lasers, have been studied. In the case where the individual semiconductor lasers have the same gain, the semiconductor laser array devices tend to oscillate laser beams with a 180.degree. phase-shift therebetween rather than with a 0.degree. phase-shift therebetween. This is because the optical intensity distribution is in accord with the gain distribution in the 180.degree. phase-shift mode rather than in the 0.degree. phase-shift mode, resulting in a high oscillation gain.
The radiation pattern, i.e., the far-field pattern, of laser beams with a 0.degree. phase-shift therebetween exhibits a single peak so that the laser beams can be concentrated into a single spot by means of optical lenses, whereas the far-field pattern of laser beams with a 180.degree. phase-shift therebetween exhibits dual peaks so that the laser beams cannot be concentrated into a single spot by any optical lens. These semiconductor laser array devices, producing laser beams which cannot be concentrated into a single spot, are inconvenient for optical coupling with other optical systems, and cannot be used as light sources for optical discs, etc. Thus, semiconductor laser array devices producing laser beams with a 0.degree. phase-shift therebetween are required.
A semiconductor laser array device which selectively oscillates laser beams with a 0.degree. phase-shift therebetween has been proposed by Japanese Patent Application No. 60-67448, wherein a waveguide structure with symmetrically branching waveguides is employed. A semiconductor laser array device has been also proposed by M. Taneya, M. Matsumoto, et al., Appl. Phys. Lett., 47(4), 1985, wherein selective oscillation in a 0.degree. phase-mode is experimentally observed.
FIG. 4 shows the waveguide structure of each of the above-mentioned semiconductor laser array devices, wherein a plurality (an odd number) of waveguides 31 are disposed in a parallel manner with a certain pitch and a plurality (an even number) of waveguides 32 are disposed, opposite the said parallel waveguides 31, in a parallel manner with a certain pitch. These parallel waveguides 31 and 32 are optically connected to each other by symmetrically branching waveguides 30. When laser beams with a 0.degree. phase-shift therebetween are propagated from the parallel waveguides 31 (or 32) to the parallel waveguides 32 (or 31) through the symmetrically branching waveguides 30, they undergo no optical loss due to the optical interference therebetween. However, when laser beams with a 180.degree. phase-shift therebetween are propagated from the parallel waveguides 31 or 32 to the parallel waveguides 32 or 31 through the symmetrically branching waveguides 30, they experience loss, to a great extent, due to the optical interference therebetween, so that the semiconductor laser array devices with such a symmetrically branching waveguide structure can attain selective oscillation. The optical intensity distributions in the 0.degree. phase-mode at the light-emitting faces of the array devices are, as shown in FIG. 4, the distributions 33 and 34 indicating that the optical intensity within each of the parallel waveguides 31 and 32 exhibits a sine curve, the peak of which decreases from the center of the waveguides to both ends. Since the optical intensity becomes maximum in the center waveguide as mentioned above, the center waveguide of the conventional semiconductor laser array devices tends to deteriorate.